The separation of components from mixtures using membranes has become an accepted form of separations technology in the literature. Processes such as reverse osmosis, dialysis, ultrafiltration, pervaporation, perstraction, forward osmosis, etc. have been described. The use of multiple membrane stages in membrane separation processes has also been described.
Thus, in U.S. Pat. No. 2,923,749, a process for separating organic compounds is described--a process especially useful for separating aromatics from non-aromatics. In the process, the feed is sent to a membrane separation zone preferably operated under a vacuum (pervaporation) to yield a permeate and a retentate. A number of permeation zones may be used. When the concentration of the components therein is suitable, permeate and retentate may be recycled to the various stages.
U.S. Pat. No. 2,947,687 describes hydrocarbon separation by the use of a permeation membrane operated under perstraction or pervaporation conditions. By using a sweep gas or by maintaining the permeate zone at sub atmospheric pressure, permeate is removed from the permeation zone. Retentate from one separation stage can be used as the feed to subsequent membrane separation zones. In the Example each permeation stage is maintained at the same vacuum level--about 100 mm Hg abs. Each successive stage is employed to remove additional quantities of the same component. See also U.S. Pat. No. 3,043.891.
U.S. Pat. No. 2,985,588 describes a membrane permeation separation technique which provides trouble free start-up and shut down procedures. In the process the permeation temperature is maintained between the softening point transition temperature and not higher than 20.degree. C. above the first order transition temperature displayed by the plastic membrane; the mixture of molecules in the feed zone is maintained in the liquid state; the mixture of molecules in the permeate zone is maintained in the vapor state; the absolute pressure in the permeate zone is maintained at less than one-half (e.g., one-tenth) of the vapor pressure normally exerted by the mixture in the permeate zone. In the Example supporting the description of the figures a hydrocarbon mixture containing 50% benzene is sent to a membrane unit at about 90.degree. C. The retentate containing about 23% benzene (the remainder being other hydrocarbons) is sent to a second membrane unit at about 110.degree. C. This higher temperature can be employed because of the lower concentration of benzene present in the feed to the second membrane unit. The retentate from this unit containing about 7.5 % benzene (the remainder being other hydrocarbons) is sent to a third membrane unit after being heated to about 120.degree. C. This higher temperature is permissible because of the lower concentration of benzene present in the feed to this third membrane unit.
In this patent temperature staging is used to separate benzene from a mixture of benzene with other hydrocarbons. The patent does not show that temperature staging can be used to fractionate a multi-component feed mixture into numerous fractions of different molecular types and different boiling points, the earlier stages containing a majority of the lighter simpler, lower boiling components and the later stages containing the heavier more complex higher boiling components.